Creatine

Creatine is good for adding some weight. I personally would suggest just keeping a good diet with very high porportionate of protein, and use some Whey protein. I have had a pretty good run with Whey protein (they carry it at Walmart now) and I have never had any issues. I would drink two shakes a day made with a ton of ice/yogurt/bananas and a little scoop of ice cream. If you follow that program of shakes/good diet and weight training it will take you no time at all to start swelling back out.